It is a well-known practice to embed various kinds of reinforcing materials in rubber to improve its strength and durability. A composite material made of rubber and metal is particularly applied to automotive tires, high-pressure rubber hoses, conveyor belts or the like and is expected to meet many quality requirements. Among them, it is a very important requirement to increase the adhesion between the rubber and the metal in a composite material to improve its durability.
This requirement is particularly strong with a steel radial tire in which are used steel cords plated with copper-zinc (brass) binary alloy as a reinforcing metal. With the expansion of highway, such a tire is expected to reveal high durability and high stability while running at high speed. For this purpose, it is very important to improve the adhesion between steel cord and rubber. In particular, a large-sized steel radial tire for trucks and buses is strongly required to maintain good adhesion for a prolonged period of time, since such a tire is used after repairing its tread portion once or even twice.
When the rubber in a composite material is vulcanized, the copper in the plating on the steel cords reacts with the sulfur in the rubber to form a sulfide at the interface between the steel cords and the rubber, thus adhering them together. Various trials have been made to improve the plating on the metal and the rubber in the composite material to increase the adhesion between the metal and the rubber. As a result, the adhesion after vulcanization, that is, the initial adhesion is being improved. But the initial adhesion tends to decrease gradually as the material is exposed to severe conditions during the service life. Thus, the adhesion durability has not yet reached a sufficient level. Therefore, the prior art composite materials have still much to be improved.
If the adhesion between the rubber and the steel cords is insufficient, the adhesion might be broken owing to heat accumulation due to hysteresis loss of rubber while the tire is rotating, causing what is called the separation between the steel cords and the rubber, thus prohibiting the tire from rotating any more.
Further, if the tire gets damaged in its tread or side wall and the damage is so deep as to reach the steel cords, moisture might infiltrate into the tire through the damaged portion, vaporize by heat generated while the tire is rotating, and invade into between the filaments of the steel cords to break the adhesion between the steel cords and the rubber, inducing the aforementioned separation. If the air in the tire contains moisture, it might infiltrate the inner liner to attack the steel cords, thus causing separation in the same manner as described above.
Various methods have been proposed to increase the adhesion between the rubber and the steel cords in a radial tire in order to avoid such a phenomenon.
One of such methods is to modify the composition of the rubber covering the steel cords. It was found out in 1950's that the adhesion between the rubber and the steel cords can be improved by adding a cobalt salt of an organic acid into the rubber. The optimum type and quantity of the cobalt salt have been investigated in various trials since then. Japanese Unexamined Patent Publication No. 60-42440 teaches to optimize the quantities of sulfur and cobalt salt of an organic salt in order to improve the adhesion. Another approach has been to use an HRH series composition containing silica, resorcin and hexamethylenetetramine.
However, if the rubber material contains an excessive amount of cobalt salt of organic acid, the quality of unvulcanized rubber might deteriorate and the adhesion might degrade as a result of long or high-temperature vulcanization or heat aging after long operation. Especially with an HRH series rubber composition, there might arise such problems as environmental contamination or rubber scorching in the production.
Another method is to improve the plating on the steel cords. For example, Japanese Patent Applications Nos. 54-127847, 54-173889 and U.S. Pat. No. 4,226,918 propose to use a ternary alloy plating comprising brass and nickel. However, the ternary alloy plating on the steel cords tends to get damaged during wire drawing because of its poor workability. Also, the addition of nickel decreases the initial adhesion between the rubber and the steel cords. Thus, this method has not yet been put into practical use. On the other hand, if the content of additives in the rubber such as cobalt salt of organic acid, silica, resorcin and hexamethylenetetramine is reduced to a small amount or zero in order to improve the quality characteristics of the rubber or to avoid the production problems, the decrease in the initial adhesion due to the addition of nickel to the plating will be marked.
The problem with the drawing of the steel cords might be solved by improving the drawing conditions such as the shape of a dice and the lubricant used. But no countermeasure has been taken to the problem of decrease in the initial adhesion due to the addition of nickel to the plating. The problem has to be attacked from the viewpoint of the compositions of both the plating and the rubber.
It is an object of the present invention to provide a composite material comprising metal and rubber which is improved in the adhesion after wet heat aging and adhesion after heat aging (hereinafter referred to as "wet heat resistance" and "heat resistance", respectively) without impairing the initial adhesion or the quality characteristics of the rubber or causing any production problems.